1. Field of the Invention
This invention relates to power subsystems and, more particularly, to split power supply design.
2. Description of the Related Art
As the performance of CPUs continues to increase, the power requirements typically increase. Underwriters Laboratories (UL) standard #60950 states that untrained end users cannot have access to power supplies with over 250 Volt-Amps (or Watts). For example, this standard limits the power at any exposed contact to a power rail from exceeding 250 Watts if an untrained end user will have access to it. Modern CPUs may only be a couple of years away from exceeding this power limit. Exceeding this power limit may pose a safety hazard to users that come in contact with the non-compliant power line.
In a typical computer system, the 12V wires and cables extending from the main power supply to the CPU may have the greatest risk of exceeding the 250-Watt limit. More specifically, the 12V lines at the front end, extending between the main power supply and the CPU voltage regulator, may pose the greatest safety hazard. Some power subsystems have a single voltage supply line connected between the main power supply and the voltage regulator. In these systems, all the power is usually concentrated into one line; therefore, the power on the line may exceed the 250-Watt limit.
Other power subsystems may have a split power supply design having two voltage supply lines. One supply line may be used to power the CPU and the other supply line may be used to power hard disk drives, floppy disk drives, cards, and other system components. In systems where the CPU requires more power than one of these lines can provide, the two voltage supply lines are typically tied together at some point before the voltage regulator. In one example, the voltage supply lines may be combined at a common point on the motherboard or via a common connector. In these systems, the point where the voltage supply lines are combined may violate the 250-Watt limit.
Some power subsystems may comply with the UL standard by including documentation that limits access to the power subsystem to trained service personnel. However, this solution is typically unacceptable for a variety of systems, such as personal computers (PC). Other systems may provide insulative shielding around the non-compliant sections of the power subsystem to prevent end user contact. One problem with this solution is that these areas may need airflow across them and the shielding may lead to cooling issues in the system. In addition, even though certain areas of the power subsystem may be shielded, contact with the power supply connector may be difficult to prevent, e.g., users may need access the connector to plug the power supply to the motherboard.
Furthermore, in some cases, power subsystems having a split power supply design may include mismatched sources, which may be connected to multi-phase voltage regulators. If these power subsystems have mismatched sources, the phase-to-phase duty cycles may mismatch, which may cause several unwanted problems, such as thermal imbalance, phase-to-phase cross conduction, and poor transient response. Having mismatched sources may even lead to incorrect current limit operation and possible phase and/or regulator shutdown. These issues may be more problematic for constant frequency voltage mode controllers, which are the most common multi-phase controllers.